1. Technical Field of the Invention
The present invention relates generally to solenoid switches (or electromagnetic switches) for controlling power supply to starter motors. More particularly, the invention relates to a solenoid switch which has an improved arrangement of a resistor that is used to limit electric current supplied to a starter motor.
2. Description of the Related Art
Japanese Patent No. 3767550, an English equivalent of which is U.S. Pat. No. 6,923,152 B2, discloses a starter for starting an internal combustion engine which includes a motor and a solenoid switch for driving the motor in two stages.
More specifically, the solenoid switch includes a pair of main contacts, a pair of auxiliary contacts, and a resistor. The main contacts are connected in parallel with the auxiliary contacts in an electric circuit of the starter for supplying electric power from a battery to the motor. The resistor is connected in series with the auxiliary contacts in the electric circuit.
During a starting operation, only the auxiliary contacts are closed in the first stage to supply limited current, which is limited by the resistor, to the motor. Consequently, the motor is energized to rotate at a low speed, facilitating establishment of an engagement between a pinion of the starter and a ring gear of the engine. As soon as the engagement between the pinion and the ring gear has been established, the main contacts are closed in the second stage to apply the full voltage of the battery to the motor, causing the motor to rotate at a high speed.
Moreover, in the solenoid switch, the resistor is arranged in a resin-made retainer so that it surrounds the radially outer periphery of a solenoid coil with an air gap formed between itself and the solenoid coil.
However, with the above arrangement of the resistor, the outer diameter of the solenoid switch is increased by an amount corresponding to the sum of the radial thicknesses of the air gap, resistor, and retainer.
Further, since the radially outer periphery of the solenoid coil is surrounded by the retainer via the resistor, it is difficult to dissipate heat generated by the solenoid coil in the radially outward direction. As a result, the temperature of the solenoid coil increases excessively, shortening the thermal withstand time of the solenoid coil.
To lower the temperature of the solenoid coil, one may consider enlarging the solenoid coil. However, this would increase the weight of the solenoid switch as well as make it difficult to minimize the solenoid switch.